The majority of cases were centered on the New Haven Green, adjacent to Yale University.

"Testing done today in New York by the federal Drug Enforcement Agency confirms that samples of the substance that caused today's overdose cases on the New Haven Green are K-2, a potent, synthetic drug that induces marijuana-like effects," said Director of Communications for New Haven, Laurence Grotheer. "DEA testing revealed no additives to the K-2 samples."

One patient didn't respond to Naloxone – a drug used to treat narcotic overdoses in emergency situations, and is “very sick”, according to Rick Fontana, the city’s director of the Office of Emergency Operations. Their current condition is unknown, but most of the overdoses and illnesses were non-life threatening.

The patients were taken to Yale-New Haven Hospital and St. Raphael's Hospital. One person has been arrested so far.

"The man was arrested within the last hour by members of the NHPD Intelligence Unit," police said in a release. "The identity of the man arrested will not be released until such time he’s been positively identified by any probable victim so as not to taint the investigation."

The Quinnipiac Valley Health District released the following signs of an overdose:

Just as emergency officials were wrapping up their press conference, talking about nearly 20 overdoses on the New Haven Green, there was a gentleman two blocks away that some of the emergency responders had to rush to as a result of another overdose. @FOX61Newspic.twitter.com/Vb3uREthJa

Two more overdoses now on the lower New Haven Green. The total, since last night, is now 23. Emergency medical officials believe it’s a combination of a synthetic weed, K2, which is been laced with something else. @FOX61Newspic.twitter.com/F6mPU8VBzN